The conductivity of a solution is generally monitored using either of two basic methods. One measures conductivity directly by maintaining a fixed voltage between two electrodes immersed in solution so that the resulting current flow is directly proportional to the conductivity. Alternately, the electrodes may be supplied with a constant current flow so that the potential between them is directly proportional to the resistivity of the solution, which is the reciprocal of its conductivity.
Close control of production electrocoat paint conductivity has been found necessary for good paint coverage of a substrate, uniform coating thickness and minimization of pinholes. High conductivity of the paint can cause excessive paint film thickness, pinholes, and a rough uneven surface appearance while low conductivity of the paint can give insufficient paint film thickness and poor "throwing power". Also, if the conductivity of the paint gets too high, ultrafilters can plug, requiring excessive maintenance.
In many applications, a simple two electrode system has sufficed. The use of alternating current of relatively high frequency is preferred for continuous monitoring of the conductivity of a liquid to inhibit the corrosion and buildup of electrolysis products at the electrode surface interface with the solution. Nonetheless, the use of the traditional two electrode conductivity meter, to measure the "batch" conductivity of an electrocoat paint bath, is associated with certain limitations. To make accurate measurements, the electrodes must be cleaned often since the fouling of the electrodes by the buildup of the paint can introduce a substantial impedance across the interface between the electrode metal and the paint solution. This affects the accuracy of the conductivity reading by indicating a much lower conductivity value than is accurate for the paint bath. Such two electrode meters are generally not commercially used on/line, i.e., in a re-circulating paint line. As discussed above, the electrodes would be quickly contaminated by the paint and frequent cleaning, as would be required to make accurate measurements, would necessitate that the paint operation be periodically interrupted to permit inspection and cleaning of the electrodes. However, the alternative to not cleaning the electrodes is error in the conductivity measurement which could have serious adverse consequences for the painting operation and the entire recirculating system.
In order to overcome problems of inaccurate readings due to fouling of two electrode systems, various four electrode systems have been developed. The outer or inner electrodes are current electrodes, generally connected to a source of alternating current, and the other pair of electrodes are voltage electrodes. A cell of this form can be used in either a constant current or a constant voltage mode. However, because negligible current is drawn from the voltage electrodes, even when fouling does occur at all the electrode surfaces resulting in additional impedence at the liquid/electrode interface, the voltage at the voltage electrodes will, within limits, remain unaffected. Thus, with the four probe cell, it is assured that the current corresponds to the voltage at the voltage electrodes, even though the voltage across the current electrodes may increase due to increased impedance arising from fouling.
Reichertz, in U.S. Pat. No. 2,599,413, teaches a constant current meter of this type for measuring the specific electrical resistivity of liquids collected from wells or muds used in connection with drilling. The meter employs four electrodes in a tube having one closed end, the outer electrodes being connected to a source of alternating current and the inner electrodes being voltage electrodes. The circuit includes an AC potentiometer which is balanced by hand, the value of the potentiometer setting at the null being proportional to the resistivity of the liquid. The voltmeter reading is, however, a measure of resistivity, which is inversely proportional to the measured conductivity. The instrument does not give a direct reading of conductivity and this is undesirable. Similarly, Carter et al in U.S. Pat. No. 2,871,445, teach a well fluid resistivity measuring apparatus employing four electrodes in a chamber, whereby the outer electrodes are current electrodes and the inner electrodes are voltage electrodes. The Carter et al invention is taught to be an improvement over the Reichertz invention in that it reads absolute resistivity while the Reichertz meter reads relative resistivity. Nonetheless, these two devices are not suitable for continuous reading of the conductivity of a liquid because repeated hand balancing is needed to obtain the readings.
Peranio, in U.S. Pat. No. 3,376,501, teaches a constant voltage cell for determining the conductivity or salinity of underground water in sand. The cell, which is lowered into the sand, comprises an electrically non-conducting tube having openings (to allow the water in) and four electrodes, the outer electrodes being current electrodes and the inner electrodes being voltage electrodes. Wilson, in U.S. Pat. No. 3,924,175 teaches a DC system for measuring the conductivity of an electrolyte. The use of the DC measurement system is taught by Wilson as being advantageous over AC systems because it does not require a precisely settable and stable high frequency oscillator for generating an a.c. current. It is further taught that polarization is avoided with the use of four electrodes and particular circuitry which allows for an incremental change in current flow between one pair of electrodes to be measured as an incremental change in polarization caused by this current flow. Either the exterior or interior electrodes may serve as reference potential electrodes and respectively either the interior or exterior electrodes pass current for creating the potential. It is taught that preferably the electrodes are arranged in a tube so that a cell constant can be determined, which eliminates the need for calibration of the cell with standardization solutions. However, a deficiency of this system is that changes in the polarization voltage associated with the voltage electrode with time would cause an error in the conductivity measurement. Barben, in U.S. Pat. No. 4,118,663, teaches a four electrode AC conductivity sensor, the inner electrodes being current electrodes and the outer electrodes being voltage electrodes. Also the electrodes may be arranged in other ways in a unitary probe structure. Barben teaches that DC current isolation is provided by certain coupling capacitors, so that the conductivity sensing circuitry is effectively isolated from a DC ground loop that might be established through the recirculation piping to the exterior control circuitry.
All of the above four probe systems have the additional disadvantage that their ability to provide accurate conductivity measurements is adversely affected by nearby AC grounds. Warmoth et al in U.S. Pat. No. 3,993,945 teach that the use of a cell having at least five electrodes allows for the conductivity of liquids, e.g., in kidney dialysis machines, to be measured on-line without error that would be otherwise caused by nearby grounds, e.g., grounded pipe works. This cell includes a tube having first and fourth electrodes which are current electrodes, second and third electrodes, between the first and fourth electrodes, which are voltage electrodes, and the fifth electrode, outside the fourth electrode, for connection to the first electrode and presenting a high impedance to the first electrode and a low impedance to the fifth electrode. Warmoth et al explain that the fourth electrode (a current electrode) is grounded and thus no current will flow between the first and fourth electrodes so that leakage current from the fifth electrode to surrounding grounded pipe works does not affect the conductivity measurement of the liquid.
It is an object of the present invention to provide an instrument having a four electrode probe which can be employed on-line in a recirculating electrocoat paint bath to provide continuous and reliable conductivity readings without the need for electrode cleaning.
It is a further object of the present invention to provide such an instrument for measuring absolute electrical conductivity of a liquid, whose ability to measure the conductivity accurately is not adversely affected by nearby system grounds or conductors. This object requires an instrument in which the flow of electrical current from the electrodes, for example to grounded metal pipe work connected to the probe, is thus inhibited or not counted in the measurement of the conductivity of the liquids.
It is still a further object of the invention to provide such an instrument which is able to measure the absolute conductivity of a liquid without the need for conductivity standard solutions to calibrate the instrument.